Ultrasonic, flow measuring devices are widely applied in process and automation technology. They permit simple determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices frequently work according to the travel-time difference principle. In the case of the travel-time difference principle, the different travel times of ultrasonic waves, especially ultrasonic pulses, so-called bursts, are evaluated as a function of the flow direction of the liquid. For this, ultrasonic pulses are sent at a certain angle to the tube axis both with, as well as also counter to, the flow. From the travel-time difference, the flow velocity can be determined and therewith, in the case of known diameter of the pipeline section, the volume flow.
The ultrasonic waves are produced, respectively received, with the assistance of so-called ultrasonic transducers. For this, ultrasonic transducers are mounted in and/or on the tube wall of the relevant pipeline section. There are also clamp-on ultrasonic, flow measuring systems. In the case of clamp-on ultrasonic, flow measuring systems, the ultrasonic transducers are pressed externally on the tube, or pipe, wall. A great advantage of clamp-on ultrasonic, flow measuring systems is that they do not contact the measured medium and can be mounted on an already existing pipeline.
The ultrasonic transducers are normally composed of an electromechanical transducer element, e.g. a piezoelectric element, and a coupling layer. Ultrasonic waves are produced in the electromechanical transducer element as acoustic signals and, in the case of clamp-on systems, led via the coupling layer to the tube wall and from there into the liquid or, in the case of inline systems, directly coupled via the coupling layer into the measured medium. The coupling layer is sometimes referred to as a membrane or diaphragm.
Transmission between the ultrasonic transducer and a corresponding measuring electronics occurs via a transmitting stage, which is embodied as a so-called push-pull stage. Starting from this transmitting stage, signal transmission of a supply voltage occurs via a resistor, respectively an impedance, to an ultrasonic transducer in the transmitting state.
Problematic is that after the last transmission pulse, the output of the push-pull stage is high resistance, which leads to a shifting of the output voltage by a value significantly above/below zero volt. This is shown in FIG. 4.